Mar
18
Researchers at Brown University Institute for Molecular and Nanoscale Innovation have come up with a new way of making thin perovskite films for solar cells at room temperature. In just five years of development hybrid perovskite solar cells have attained power conversion efficiencies that took decades to achieve with the top-performing silicon semiconductor materials used to generate electricity from sunlight.
The first perovskite cells introduced in 2009 managed an efficiency of only about 4 percent, far below the 25 percent efficiency boasted by standard silicon cells. But by last year, perovskite cells had been certified as having more than 20-percent efficiency.
New Process Making Pervoskite Solar Cell Samples. Click image for more info.
Perovskites, a class of crystalline materials that when made into films are excellent light absorbers and are much cheaper to make than the silicon wafers used in standard solar cells.
There are a number of different ways to make perovskite films, but nearly all of them require heat. Perovskite precursor chemicals are dissolved into a solution, which is then coated onto a substrate. Heat is applied to remove the solvent, leaving the perovskite crystals to form in a film across the substrate.
Nitin Padture, professor of engineering and director of the Institute for Molecular and Nanoscale Innovation at Brown University said, “People have made good films over relatively small areas – a fraction of a centimeter or so square. But they’ve had to go to temperatures from 100 to 150ยบ C, and that heating process causes a number of problems.”
For example, the crystals often form unevenly when heat-treated, leaving tiny pinholes in the film. In a solar cell, those pinholes can reduce efficiency. Heat also limits the substrates on which films can be deposited. Flexible plastic substrates, for example, cannot be used because they are damaged by the high temperatures.
Yuanyuan Zhou, a graduate student in Padture’s lab, started looking for a way to make perovskite crystal thin films without having to apply heat. He came up with what is known as a solvent-solvent extraction (SSE) approach.
The team’s paper has been published in the Royal Society of Chemistry’s Journal of Materials Chemistry A showing the technique produces high-quality crystalline films with precise control over thickness across large areas.
Zhou’s method has perovskite precursors dissolved in a solvent called NMP and coated onto a substrate. Then, instead of heating, the substrate is bathed in diethyl ether (DEE), a second solvent that selectively grabs the NMP solvent and whisks it away. What’s left is an ultra-smooth film of perovskite crystals. With no heating involved, the crystals can be formed on virtually any substrate, even heat-sensitive polymer substrates used in flexible photovoltaics.
Another advantage is that the entire SSE crystallization process takes less than two minutes, compared to an hour or more for heat-treating. That makes the process more amenable to mass production because it can be done in an assembly line kind of process.
Zhou’s work looks even better because the SSE approach also enables films to be made very thin while maintaining high quality. Standard perovskite films are generally on the order of 300 nanometers thick. But Zhou has been able to make high quality films as thin as 20 nanometers. The SSE films could also be made larger, several centimeters square without generating the disabling pinholes.
Zhou said, “Using the other methods, when the thickness gets below 100 nanometers you can hardly make full coverage of film. You can make a film, but you get lots of pinholes. In our process, you can form the film evenly down to 20 nanometers because the crystallization at room temperature is much more balanced and occurs immediately over the whole film upon bathing.”
Initial testing of cells made with SSE films showed conversion efficiency of over 15 percent. Solar cells based on semitransparent 80-nanometer films made using the process were shown to have higher efficiency than any other ultra-thin film.
One has to think that a lot of research and development folks are going to be reading this team’s study paper.
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